The present invention relates to keratoplastic instruments for performing intraocular surgery and more particularly to such instruments and methods for performing penetrating keratoplasty utilizing corneal transplantation. It further concerns surgical apparatuses or trephines for precisely cutting out a small piece of the cornea.
Erasmus Darwin may have been the first to suggest in 1794 that a piece of a healthy cornea from a donor could be transplanted for a piece of unhealthy cornea in the recipient patient. This procedure can be used as when the unhealthy cornea has become opacifed or affected. In performing this transplant operation a corneal trephine is currently used to cut a circular disk or button from the corneal tissue.
Many designs for trephines are known and perhaps the simplest is a cylinder having an ultrasharp circular cutting edge at one end. This cylinder is positioned carefully over the cornea to be excised and with a downward pressure and a twisting or rotating of the cylinder the cutting edge will make a generally circular cut in the cornea. The cut can be made 360.degree. all the way through the cornea into the interior chamber or an incomplete or partial cut can be made with the trephine and then the partial cut completed with curved corneal scissors.
In the early 1960s Jack Guyton, M.D., and John Balian, M.D., developed a trephine which would more effectively cut straight sides in the cornea. The underlying principle of their trephine was to provide suction to hold the eye up so that the trephine blade would penetrate the cornea without pushing down on the eye. By pushing down on the eye, non-parallel sides to the graft are invariably created. Their device comprised a floor-mounted motor driven corneal trephine in which a suction ring was provided to support the cornea and it was a rather cumbersome and complex device.
Many other simpler vacuum suction ring trephines have since been developed, and two of the trephines currently being sold which employ vacuums to secure the mechanism to the eye are the Hessberg-Baron and the Caldwell, both from Jed-Med, of St. Louis, Missouri. However, even these simpler and more developed vacuum type trephines suffer from many disadvantages. For example, the surgeon is able to see the cornea only through the narrow cylinder of the trephine in most of these designs either with this unaided eye or through a microscope. This makes it difficult to ensure the correct placement of the trephine on the cornea and also to orient the trephine at right angles to the plane of the iris. Further, it is difficult for the surgeon or his assistant to ascertain the depth of the cut which has been made with the known suction type trephines since only the inside area of the cut is visible through the narrow trephine cylinder. Since it is the tendency especially of the novice surgeon to not make a deep enough cut, and since this can often not be ascertained without removing the trephine from its position on the eye, the surgeon frequently must try to reposition the trephine in the exact same spot and with the exact same orientation as his initial cut to complete the cut. Of course, this is difficult if not impossible to do consistently. Other keratoplastic instrument designs are illustrated in U.S. Pat. No. 3,074,407, whose contents are hereby incorporated by reference in their entirety. Those designs though are rather complicated and do not appear to be easy to use in practice. They also do not provide for the repositioning of the trephine blade in the blade holder to complete an incomplete cut.
If the trephine is tilted relative to the plane of the iris then a generally oval wound or cut is produced. The greater the tilting, the greater (in an exponential relation) the difference in the lengths of the major and minor axes of this oval configuration result. The greater the amount of the tilt, the larger the resulting asphericity and resulting astigmatism.